(1) Field of the Invention
The present invention relates to a substation apparatus for satellite communications in which communication between a master station and a plurality of substations is achieved via a communication satellite, and more particularly, to a substation apparatus which enables a substation to access a master station in satellite communications wherein a vast amount of data is transmitted in burst mode between a master station equipped with a host computer and a plurality of substations each equipped with a terminal processing device.
Recently digital satellite communication techniques have been investigated and actively put to practical use with an increasing demand for non-telephonic services such as data communication and pictorial communication and the development of digital devices such as computer and LSI. One of topical digital satellite communication procedures is a TDMA (Time Division Multiple Access) method. The substation apparatus of the present invention permits a substation to access a master station where a large amount of burst data is transmitted according to the TDMA procedure.
(2) Description of the Related Art
When a VSAT (Very Small Aperture Terminal) station as a substation accesses a HUB station as a master station for data communication, the following access methods are conventionally used:
(a) Time slots are previously assigned to individual VSAT stations, and each VSAT station transmits data to the HUB station within the time slot uniquely assigned thereto. PA1 (b) Each VSAT station immediately transmits data to the HUB station within the time slot as soon as the data is generated. PA1 (c) Each VSAT station makes a reservation for transmission with respect to the HUB station when transmitting data is generated, and transmits the data to the HUB station within a time slot assigned thereto by the HUB station.
The method (a) is suited for the case wherein almost equal amount of transmitting data is usually generated in each of the VSAT stations.
The method (b) is suited for the case wherein data is generated in burst mode in each VSAT station. In order to lessen the probability that data transmitted from the VSAT stations will collide with one another, the transmissible amount of data is set to a value greater than the amount of actually transmitted data. As a result, the frequency band occupied by the system inevitably broadens, compared with the scale of the system.
The method (c) also is suited for the case wherein a large amount of burst data is generated at each VSAT station. In this method, however, a reservation for transmission must be made before data is transmitted.
Thus, the method (a) is not suited for transmitting a large amount of burst data, while although the method (b) is suited for burst transmission of data, the occupied frequency band greatly broadens with increase in the amount of burst data, making the method unpractical. Accordingly, the method (b) cannot be said to be suited for transmitting a large amount of burst data. In the method (c) which permits transmission of a large amount of burst data, since a reservation for transmission must be made prior to data transmission, actual data transmission is delayed usually by about 0.5 second due to the reservation.